The invention relates to a polarization scrambler comprising a controllable optical retardation element of which the retardation is controllable as a function of time by a control arrangement in such a way that with a given state of polarization (SOP) of the light entering the polarization scrambler varying states of polarization occur at its output in such a way that, averaged in time, the light intensities emerging in each SOP are at least substantially equal.
Such an arrangement is known from Electronics Letters, Vol. 23, No. 12, pp. 634 and 635.
In optical transmission systems a transmission light source, particularly a laser, may precede optical elements whose behaviour is dependent on the state of polarization (SOP) of the light. For example, the transparency of a beam splitter may depend on the SOP of the input light. Unambiguous transmission ratios are achieved if a constant SOP throughout a transmission path could be guaranteed. If at all, this could only be achieved by means of cumbersome control processes.
Particularly when light waveguides (i.e. optical fibres) which are not specifically polarization maintaining fibres are arranged in the transmission path, it is inevitable that these conventional light waveguides induce SOP variations due to their properties of birefringence as a function of time.
In the case mentioned in the opening paragraph these difficulties should be eliminated by passing input light having a known SOP via a polarization scrambler so that light having a periodically varying SOP as a function of time is produced at its output. It is true that less light power than is possible in the case of optimum adaptation is available at the output of the polarization scrambler in a given polarization plane, but this light power, averaged in time, is constantly transmitted even when the polarization rotations of the transmission path fluctuate accidentally.
Special problems occur in backscattering meters or reflectometers with which the attenuation behaviour of monomode light waveguides is measured (OTDR). Since the backscattering signals in monomode light waveguides are much smaller than, for example in multimode light waveguides, a heterodyne reception is advantageous. The backscattering signals are then superimposed with a local oscillator beam (LO) having a different light frequency. The intermediate frequency signals produced are analyzed. These signals can provide information when the SOPs of the LO and the backscattering signals are in a fixed relationship and, in the ideal case, are equal. Such a presumption is not given due to the fact that the SOP is influenced in an accidentally varying manner by the light waveguide to be measured.
The laser light used for the measurement can be depolarized by means of a depolarizer. (Compare Optical and Quantum Electronics 15, pp. 281-287). If a so-called pig-tail were used for this purpose, in which the states of polarization of the light components having different wavelengths are naturally attenuated to a different extent, extremely long pig-tails would be required for narrow band transmission light (for example 100 kHz).
When using a polarization scrambler of the type described in the opening paragraph it is a prerequisite that the SOP of the input light is invariable and that the position of the polarization scrambler is exactly associated therewith. Such a polarization scrambler must thus be arranged at the area where the SOP is still practically stable, i.e. particularly at the output of a laser.